An existing LTE system uses a Cell-specific Reference Signal (CRS) for detection and demodulation of a downlink channel. As described in the 3GPP TS36.211 (v8.5.0) protocol, the cell-specific reference signal covers the entire bandwidth of a cell and is transmitted at the same power, and all the users served by the cell know the cell-specific reference signal of the cell. Therefore, the users generally can use the cell-specific reference signal as channel estimation.
Generally a cell-specific reference signal used by a user for channel detection includes both a cell-specific reference signal in a resource block thereof and a cell-specific reference signal beyond the resource block thereof as a part of a reception demodulation scheme in order to improve the quality of data reception. As illustrated in FIG. 1, a white blank square box represents a resource unit of data, a square box with netted patterns represents a resource unit of a cell-specific reference signal, a gray zone represents a resource block allocated to a user, and a square box denoted with 1 represents a resource unit of a cell-specific reference signal in the resource block, square boxes denoted with 2, 3 and 4 represent resource units of cell-specific reference signals adjacent to the resource block. For data demodulation, a set of cell-specific reference signals that can be used by the user for channel estimation includes signals of the resource units denoted with 1 and 2, signals of the resource units denoted with 1 and 3, signals of the resource units denoted with 1, 2 and 3, signals of the resource units denoted with 1, 2, 3 and 4, etc. A corresponding two-dimension filtering algorithm can be found in the article Robust Channel Estimation for OFDM Systems with Rapid Dispersive Fading Channels by Ye Li, L. J. Cimini, and N R. Sollenberger in IEEE Trans. Commun., vol. 46, No. 7, pp. 902-915, July 1998. However, channel estimation may suffer from an error when the resource unit of the cell-specific reference signal beyond the resource block is corrupted by a cell-specific reference signal transmitted from a relay station.
As specified in the latest relay protocol, a transparent relay will be included in the LTE-A specification, that is, the relay will transmit a cell-specific reference signal in the same resource unit as a base station. The relay station will transmit a cell-specific reference signal and a physical downlink control channel over the entire bandwidth for demodulation at a relay user.
As described previously, a cell-specific reference signal used by a user for channel estimation typically includes a cell-specific reference signal in a resource block thereof and a cell-specific reference signal beyond the resource block thereof in order to improve the performance of channel estimation. As illustrated in FIG. 2, a user of a base station receives both a cell-specific reference signal from the base station and a cell-specific reference signal from a relay station. When downlink frequency bands of the base station and the relay station are adjacent, a cell-specific reference signal used by the user to demodulate received data may include the cell-specific reference signal transmitted from the base station and the cell-specific reference signal transmitted from the relay station, both of which overlap with each other, thus degrading the performance of reception demodulation.